1. Field of the Invention
This invention relates to video signal processing circuits wherein automatic control of the gain and pedestal levels are provided.
2. Description of the Prior Art
Appropriate video standards have been established for sync amplitude, pedestal height and peak white and black excursions in order to ensure compatibility and uniformity in the television industry. Video signals such as those produced by video cameras are generally adjusted to comply with these standards during the camera "set-up" procedure. However, for various reasons, the resultant composite signal may not conform to those standards due to varying light conditions, scene contrast, etc. For example, the video cameras may be "remoted" such that manual adjustments or readjustments cannot be made. Under such conditions, if the signals produced by the cameras are directly recorded and subsequently reproduced upon playback, the reproduced signals may not conform to designated standards. Reproduced signals corresponding to such improperly reproduced information must be further processed to provide the requisite signal parameters. Accordingly, it has become useful to provide means for processing the signal to restore these parameters to the standard values.
Although a simple gain control circuit can adjust the entire composite video signal such that the maximum excursion, i.e., the sync pulse amplitude or maximum carrier level, is normalized to a desired level, more complex circuits are required to independently adjust the pedestal height and the amplitude of the active video excursions. Furthermore, since a change in the pedestal level results in a change in the peak white level, and vice versa, some way to compensate for such interactions must be provided.
It has become common practice to continuously provide such adjustments to the pedestral height and video excursions by analog feedback techniques. In such practices, the pedestral height variation is attained by inserting a pedestal correction signal during the active video period (during unblanking) while the video excursions are controlled by an automatic gain control loop. The appropriate peak black and peak white levels are detected, compared with the desired levels and the resultant error signals are used to control the appropriate adjustments. The interaction of the two feedback loops is often overcome by designing the response time of the automatic pedestal adjustment loop to be significantly slower than that of the automatic gain control (AGC) loop.
However, such analog control loops are incapable of providing adequate control in certain situations. For example, significant changes to both the pedestal level and the video level can occur within a single field. Thus, if the response time of the respective loops is designed to be sufficiently slow so that "shading" occurring within a few fields is minimized, then the corresponding automatic control functions are so slowed down as to appreciably reduce the usefulness of the device. Similarly, a sudden reduction in the video level from bright to dark will cause the AGC action to drive the dynamic range of the video excursions to full level, thereby resulting in a nonuniform contrast during that field. Such control loops are also incapable of providing a fixed gain over an indefinite period, i.e., to "freeze" the gain where it is.
In contrast to such analog feedback control loops, a variety of digital feedback loops for controlling video signal levels are also known. In U.S. Pat. No. 3,790,706 (Gubala), an automatic video contrast control circuit is disclosed in which the peak white and peak black portions of the picture information portion of a composite video signal are detected (the black and white detectors being disabled during blanking intervals) and are used to ensure that the video signal within a given field is expanded to provide a uniform and maximum contrast between these two levels. To so do, the picture information portion of the video signal is digitized and the difference between the peak white and peak black is divided across a voltage divider network to control each digitized portion of the video signal. However, such a system fails to provide any corrective adjustment of the pedestal level.
U.S. Pat. No. 3,670,100 (Briggs and Ward) discloses a system for automatically establishing voltage levels in a camera output signal. In this system, the output level is sensed and compared with a reference signal to establish a digital signal indicative of whether the sensed signal is greater or less than the reference. The digital signal is then converted to a corresponding analog signal and fed back to control the output signal. The digital comparator is preferably activated once per field. No provision is made for allowing signals within a prescribed range to remain unchanged, nor is there any suggestion for independently adjusting both the pedestal level and the peak white level.